1. Field of the Invention
The present invention relates to a detection apparatus for electrically detecting the presence of an object, an image forming apparatus in that the presence of a sheet is detected using the detection apparatus, and a sheet transport apparatus.
2. Description of the Related Art
As disclosed in Japanese Patent Laid-Open No. H10-087115, a photo-interrupter has been used for detecting a sheet being conveyed within an image forming apparatus. A general photo-interrupter is shown in FIG. 8. In a photo-interrupter 501, a light-emitting diode 514 (light emission unit) and a photo-transistor 515 (light receiving unit) are arranged to oppose each other with a space 502 therebetween. The photo-interrupter 501 can electrically detect the presence of a masking object in the space 502 to be output.
An internal circuit of the photo-transistor 515 is shown in FIG. 9. In the photo-interrupter 501, the light-emitting diode 514 and the photo-transistor 515 are arranged. An anode terminal (power supply) 511 is connected to the anode of the light-emitting diode 514. A cathode terminal (GND) 513 is connected to the cathode of the light-emitting diode 514 and the emitter of the photo-transistor 515. A collector terminal (detection output) 512 is connected to the collector of the photo-transistor 515.
When the light emitted from the light-emitting diode 514 enters the photo-transistor 515, the photo-transistor 515 is turned on to reduce the corrector terminal to an L-level. When the light emitted from the light-emitting diode 514 is shielded, the photo-transistor 515 is turned off so that the collector terminal becomes an H-level due to the pulling-up of a circuit connected thereto. In such a manner, for operating the photo-interrupter for electrically outputting the detection of the presence of the masking object in the space 502, the connection with three wires is necessary.
FIGS. 10A and 10B illustrate that the photo-interrupter is used for detecting a sheet together with other mechanical members in an image forming apparatus. A mechanical flag 503 is rotatable about a rigid shaft 505. In FIG. 10A showing the absence of a sheet, one end of the mechanical flag 503 is raised by a spring 504. In this case, since the light in the space 502 is not shielded, the output of the photo-interrupter 501 becomes the L-level.
In FIG. 10B showing the presence of a sheet, the mechanical flag 503 is pushed down into the space 502 by a sheet 109, so that the light of the photo-interrupter 501 is shielded and the output of the photo-interrupter 501 becomes the H-level.
FIG. 11 is a control block diagram illustrating when the photo-interrupter is used for detecting a sheet being conveyed in an image forming apparatus, and includes a control unit 121 having a CPU and an I/O port for control, a driver unit 122 for executing various operations based on the control of the control unit 121, and photo-interrupters 123a-123z. For actuating the interrupters 123, each interrupter is connected to the control unit 121 or the driver unit 122 with three wires for each.
Since a number of the interrupters 123 are used in the image forming apparatus, it is necessary to connect the many interrupters 123 to the control unit 121 and the driver unit 122 with wires. Thus, it is required to provide, in the image forming apparatus, a number of wires long enough to reach the interrupters 123, which are arranged all over the image forming apparatus, from the control unit 121.
The existence of many long wires increases the possibility of generating the contact failure of connectors and the wire breaking, causing troubles. With increasing number of wires, the cost of the wire itself and the cost for wiring the image forming apparatus are increased.